Civil-defense sirens are designed to warn a surrounding population about a dangerous situation that is about to occur. A civil-defense siren typically generates a single sound that is audible over a geographic area surrounding the siren. Such sirens were originally developed during World War II to provide warnings about air raids. They are now often used to warn of approaching tornadoes or severe storms. Civil-defense sirens are thus sometimes called air-raid sirens or tornado sirens. Existing civil-defense warning systems such as sirens can suffer from shortcomings under which warnings are not provided, or fail to convey important information about a threat. There is a need for an improved technical infrastructure to enhance civil-defense warnings.